Earth's water storage capacity increases with age

A rare lawsonite eclogite in Guatemala, which demonstrates exceptional water storage capabilities.

The ability of the Earth to recycle water between its surface and inner layers is affected by the age of the subduction zone - formed when gigantic pieces of the Earth's crust and uppermost mantle collide, according to an international team of researchers.

A subduction zone forms when two tectonic plates collide and one is forced beneath the other. During the process, a variety of rocks are subjected to changes in pressure, temperatures and chemical environment, and undergo metamorphosis as a result. This process is important for recycling water, along with critical elements, such as strontium, uranium, thorium and lead, between the Earth's surface and its deep interior.

"Lawsonite is important for recycling water deep beneath the Earth's surface."

Professor Tatsuki Tsujimori, Tohoku University

One such metamorphic rock that forms at high pressure is lawsonite eclogite, which plays a crucial role in storing water in subducting plates and can carry large quantities of water to the deeper mantle. Lawsonite is not commonly found in fossilised subduction zones on the Earth's surface, which provided the scientists with further questions regarding current understanding of how water is stored in subduction zones.

Scientists have traditionally thought that oceanic crust turns into lawsonite eclogites in cold subduction zones, based on models and experiments that point to lawsonite being a common mineral in cold geothermal regimes. Yet, the opposite is true.

To investigate this puzzle, a team lead by Dr David Hernández Uribe from the University of Illinois Chicago in the US, and Professor Tatsuki Tsujimori from Tohoku University, Japan, used the latest modelling techniques to simulate rock formation at different lifetime stages of a subduction zone. The group revealed that in the early stages of a subduction zone - under six million years of age, oceanic crust does not turn into lawsonite eclogites, but over longer time periods - 12-33 million years - it does.

"We found that the formation of lawsonite eclogites depends on how mature the subduction zone is," says Tsujimori. "Lawsonite is important for recycling water deep beneath the Earth's surface only in mature subduction zones. In younger zones, it doesn't play as big of a role as previously thought."

Details of the findings were reported in the journal Geology, and will aid scientists in the understanding of water and mass recycling in tectonic settings. In summary, tectonic plates subducting early in their subduction zone timeline will not carry as much water as plates subducting in the more mature stages.